The present invention relates to a process for producing deep-drawing cold rolled steel strips by continuous annealing.
Cold rolled steel strips are very often used in the manufacture of cold-formed articles, such as press-formed automobile parts, and the strips are thus required to be soft and to have an excellent press-forming property.
Conventionally, aluminum-killed steels have been usually treated by a box annealing process for production of the deep-drawing cold rolled steel strips. However, the batch annealing process has the critical disadvantage that the process takes a long period of time to perform, and hence considerably lowers the production efficiency.
Therefore, much attention has been paid to new arts such as continuous annealing process, aiming at the production of deep-drawing cold rolled steel strips, and in recent years some continuous annealing processes have been in actual practice for production of cold rolled steel strips.
In the conventional continuous annealing arts, the steel is subjected to rapid heating, a short time of soaking and then rapid cooling. Therefore, when an Al-killed steel or an ordinary low carbon steel is treated by a conventional continuous annealing, the resultant steel has a small grain size and is hard, showing an inferior r value which is a parameter of the deep-drawability of steels as compared to that obtained by a box annealing process, hence failing to provide a deep-drawing cold rolled steel strip which can be satisfactorily press formed.
For eliminating the above disadvantages, it has been proposed that the steel strip after hot rolling is coiled at a high temperature not lower than 700.degree. C. and subjected to cold rolling and then a continuous annealing process. However, the high-temperature coiling causes difficulties in acid pickling, and surface defects, such as ridging, which appear when the resultant cold rolled steel strip is worked, as well as deterioration of ductility due to formation of massive carbides.
Then for eliminating the disadvantages of the high temperature coiling, an art has been proposed for producing a deep-drawing cold rolled steel strip by a continuous annealing process without adopting a high-temperature coiling step, as disclosed in Japanese Patent Publication No. Sho 51-29696, according to which a soft cold rolled steel strip can be produced from an Al-killed steel containing boron by continuous annealing even with a low temperature coiling at about 650.degree. C.
However, the B-containing Al-killed steel, as disclosed in the above mentioned Japanese Patent Publication, usually contains nitrogen in an amount as about 0.005 to 0.0065% and therefore, it is essential that the steel contains at least 0.0020% B. According to the disclosure of this prior art, the addition of boron has a subsidiary harmful effect to degrade the r value. This has been the critical problem of the conventional B-containing Al-killed steel.
Deep-drawing steel strips for press forming are required to have material qualities, in addition to a high r value, such that an excellent shape can be obtained by the press forming and that they have excellent stretchability. For these qualities they are required to have desirably a low yield point and a large elongation.
However, when the B-containing Al-killed steels are subjected to continuous annealing, they are often found to be hard and have a high yield point and a low elongation, although the steel can be softened to some degrees by the continuous annealing. Therefore, it has been found to be difficult to produce a satisfactory deep-drawing steel strip with consistency from the B-containing Al-killed steel by continuous annealing.